Capillary electrophoresis (CE) or specifically capillary zone electrophoresis (CZE) is a known efficient separation technique useful for fast, high resolution and efficient separation of charged species in solutions. High resolution in CZE technique requires the introduction of a small volume sample, i.e., in the range of 5 to 50 nl.
There are a number of methods for injecting such volume of sample into the capillary column. The two principal sample injection techniques are electrokinetic and hydrostatic injection as described in the following articles: Jorgenson, J. W. and Lukacs, K. D., Science, 1983, v. 222, p. 266-272, Wallingford R. A., and Ewing A. G. Anal. Chem., 1987, v 59, p. 678-681.
Electrokinetic injection is performed through the pumping activity of electroosmosis and electrophoresis. Samples are introduced at one end of a buffer filled capillary and, stimulated by an applied high-voltage electric field, move towards the other end of the capillary tube. Samples are usually injected at the positive high-voltage end due to electroosmotic flow that carries solutes, regardless of charge, toward the negative electrode. This injection technique, while simple and easily controlled, can affect the introduced quantities of different sample components since some sample species will have higher electrokinetic mobilities than other species, (see Huang, X., et al., Anal. Chem., 1988, v. 60, p. 375-377).
Hydrostatic injection is characterized by physically introducing samples in the capillary and might be referred to as suction, pressure, or gravity injection. In this method one end of the buffer filled capillary tube is removed from the buffer reservoir and introduced into a sample solution which is raised vertically above the capillary outlet. This height difference between the level of the sample solution and the buffer reservoir yields a hydrostatic pressure which siphons sample into the capillary. Alternatively, for automatically introducing a sample into the capillary, a vacuum can be applied to the end of the capillary tube. However, hydrostatic injection increases the zone broadening effect due to laminar flows generated during the injection, and influences the separation efficiency in zone electrophoresis. Comparison of electrokinetic and hydrostatic injection techniques indicate that electrokinetic introduction is preferably because it is simpler and produces less zone broadening.